Conventionally, an internal combustion engine ignition system for igniting an air-fuel mixture in an internal combustion engine used as an automobile engine or the like is formed to include an ignition coil for generating an ignition voltage, and a spark plug for generating a spark discharge by the ignition voltage applied to the spark plug.
Generally, the ignition coil includes a coil body portion having a primary winding, a secondary winding, and a coil core for magnetically coupling the two windings. The ignition coil is configured so that an ignition voltage is generated between opposite ends of the secondary winding by variation in the magnetic flux density of the coil core in accordance with the conduction/interruption of a current flowing in the primary winding. The ignition coil is electrically connected to the spark plug through a wiring cable (current-conduction path) such as a high tension cable or the like, so that the ignition coil applies the ignition voltage to the spark plug.
Incidentally, an intake/exhaust valve drive mechanism having a cam shaft or the like for driving an intake valve and an exhaust valve is provided in an internal space formed between an upper portion of a cylinder head having the spark plug fixed thereto and a cylinder head cover. Lubricating oil for operating the intake/exhaust valve drive mechanism smoothly is circulated.
On the other hand, the spark plug has a structure in which a spark discharge is generated by application of a high voltage. If lubricating oil is deposed on the spark plug, there is a possibility that the spark discharge cannot be generated normally because of influence of electrical leakage or the like. Therefore, a cylindrical plughole pipe formed to have an inner diameter enough for the spark plug to pass through is provided in the internal space formed between the cylinder head and the cylinder head cover to thereby secure the installation space of the spark plug and isolate the spark plug from the lubricating oil. Incidentally, the plughole pipe is fixed to the cylinder head in a state in which the axial direction of the plughole pipe becomes one and the same as the axial direction of the spark plug.
After the spark plug is mounted in the installation space formed by the plughole pipe, the aforementioned wiring cable is wired between the ignition coil and the spark plug to thereby electrically connect the ignition coil and the spark plug to each other.
When the ignition coil and the spark plug are connected to each other through the wiring cable such as a high tension cable or the like, there is however a problem that as the distance between the ignition coil and the spark plug increases, the resistance value of the wiring cable increases and the loss in the wiring cable increases because of the influence of external noise or the like. When the loss in the wiring cable becomes high in this manner, energy for generating the ignition voltage is wastefully consumed by the wiring cable. In some cases, there is fear that the spark discharge may not be generated because of shortage of the voltage value applied to the spark plug.
Use of a structure (so-called direct ignition type plug) in which the ignition coil and the spark plug are directly connected to each other is effective in solving the problem. In order to use this structure, it is however necessary to secure the installation space of the ignition coil in the internal space formed between the cylinder head and the cylinder head cover. Because the intake/exhaust valve drive mechanism or the like as well as the spark plug is provided in the internal space formed between the cylinder head and the cylinder head cover as described above, it cannot be said that securement of the installation space for installing the ignition coil in the cylinder head is easy.
Therefore, an object of the present invention is to provide an ignition coil which has a structure to make it possible to connect the ignition coil to a spark plug directly and in which an installation space need not be changed largely when the ignition coil is mounted in a cylinder head, and to provide an internal combustion engine ignition system having the ignition coil.